With extensive use of portable electronic devices in the areas of communication, computer, and consumer electronic products, etc., there is an increasing need for power management integrated circuits or ICs (e.g., DC-to-DC switch power supplies, AC-to-AC switch power supplies, etc.). Further, as electronic systems become more complicated, better performance for both sequencing and smoothness during the start-up or powering on process of such power management circuits becomes more important. For example, in a relatively complicated electronic system (e.g., a laptop), a plurality of power supplies may be utilized, each of which may supply multiple loads (e.g., digital chips in the electronic system) simultaneously.
In addition, a power voltage of each digital chip may need a relatively large filter capacitor to obtain a smooth power voltage to maintain a proper operating condition. Generally, a starting switch is used to reduce standby loss, and different power voltages should have different power sequences in order to guarantee a normal start-up for the system, and thus such starting switches should also accordingly regulate the start-up delay time. Because the voltage of the filter capacitor is zero at the beginning of the start-up process, a heavy inrush current may occur due to the instantaneous surge to the power supply from the filter capacitor. For example, the filter capacitor for a digital chip may be large enough to generate an inrush current of as much as 100 A. As a result, it may be necessary to limit the inrush current during the start-up process in order to prevent damage to the power switch of the power management circuit. However, abnormal operation of the switch power in view of the huge inrush current can occur at the moment of powering on the voltage regulator or power management chip.